1. Field of the Invention
The present invention relates to a spinal fixation apparatus which can correct and immobilize injured or deformed human spine; and, more particularly, to a spine fixation apparatus for easily carrying out an operation on the spine, by using segment flexible rods and a transverse link which are made from a shape memory alloy, so that the rod and the transverse link can elongate in different shapes with flexibility.
2. Description of the Related Art
Typically, the vertebra typically consists of 24 bones (except for sacral vertebra). They are connected to each other through joint segments and there are discs between the joint segments. By this structure, the vertebra has man's posture kept and a shock absorbed. Also, the vertebra is essential to exercises and protects all internal organs from an external shock. However, the vertebra of the spine can be injured or wrenched by external environments, abnormal postures for a long time and anaplastia and so on, which may induce a serious back pain by pressing the nerve system passing through the spine.
Patients, who have been injured in a part of his spine, cannot take their activities in daily life, because the injured part of the spine is compressed by adjacent other parts of the spine. This spinal disorder requires a surgical intervention to treat the pain which is induced in nerve root by the compression and unstableness of intervertebral joints.
As shown in FIG. 1, a conventional spinal fixation apparatus includes a plurality of pedicle screws 200 which are respectively inserted through the pedicle into injured or deformed vertebra, wherein each of the pedicle screws 200 has a head 201 formed at a top portion thereof, wherein the head 201 has a U-shaped rod passage 201a and a female thread 201b is formed on an inner surface thereof, and wherein a thread 202 is formed below the head 201 in order that the pedicle screws 200 can be implanted into the vertebra; a pair of longitudinal rods 250 which are located in both laterals of the spine and coupled to the pedicle screws 200 for preventing a movement of the vertebra; a number of set screws 300, each of which has a male thread and a wrench hole 300a on an upper surface thereof, being inserted into the rod passage 201a of the head 201 of the pedicle screw 200 for preventing a movement of the rod; and a transverse link 400 for holding the longitudinal rods 250.
In the conventional spinal fixation apparatus, the rods and the pedicle screws will be described in short, referring to the accompanying drawing.
As shown in FIG. 2, the thread 202 of the pedicle screws 200 is implanted into the vertebral body 500. The rod 250 is then put into the rod passage 201a. In this situation, the set screw 300 is joined to the female thread 201b of the rod passage 201a. 
Also, the set screw 300 is joined to the rod passage 201a, by inserting it into a groove 300a and turning it with a wrench on the top thereof. As a result, the bottom side of the set screw 300 compresses the top of the rod 250 within the rod passage 201a of the pedicle screws 200.
According to this assembly, the rod 250 is definitely tightened up on the pedicle screw 200 to correct the diseased or injured vertebral body 500. The both ends of the transverse link 400 are coupled to the pair of the rods 250 so that the transverse link 400 is laid across the rods 250 which is connected to the pedicle screws 200. When a patient gets to do a wrenched action toward his left or right sides, the transverse link 400 prevents a rotation and migration of the rod 250.
In the conventional spinal fixation apparatus, the rod 250 functions as a basic element to correct the vertebra. Therefore, the material of the rods or the properties of the rod, such as elasticity, has a large effect on human body, after it is inseparably fused together with the vertebra. The rod 250 itself is not elastic because it is made from a titan alloy for medicine. It is very difficult to keep the line of his lumber normal since the vertebra segment is fused together with the rod to correct the vertebra bodies.
Also, after the vertebra is integrally fused together with the rod, the weight is concentrated upon the upper segment or the lower segment so that it will cause another vertebra stegnotic or instability of a lumbar vertebra within a few years after the vertebra fusion.
In particular, it has some problems in that the rod can be broken and buried in the vertebra, when a shock is inflicted upon the lumbar vertebra.
The rod is made to have a normal spinal curvature shape without being related to a specific spinal shape of individual. That causes the difficulty of standardizing each single products and making various shapes, thereby increasing the cost of products. Also, the rod structure induces a series of bottlenecks in connecting the pedicle screw to the rod because the rod has a straight shape. That is, the individual spinal shape is different from each other. Therefore, if the pedicle screws are not fixed uniformly between the segments, it was very difficult to install the straight rod on the pedicle screws. This is caused by the physical properties of the rod. In this case, an operator has to adjust a distance and direction of the pedicle screw based on the location of the rod, by making the pedicle screw slanted. Also, the operator has to adjust an angle of the pedicle screw's head, using the polyaxial type screw, which can freely rotate a head around the screw in a range of predetermined angle and set the rod up thereon. The operating work using the conventional vertebra fixation apparatus needs accuracy, because he must correctly grasp the location to install the pedicle screw and then have to make a hole in vertebra with a burden on the surgeon. Also, it takes a lot of times to make the rod based on a curved shape of patient's vertebra and to set the location of pedicle screw.
To solve the problems of the conventional vertebra fixation apparatus, the various types of the rods has been provided with elasticity,
An example of an elastic rod for connecting the pedicle screws is illustrated in Korean utility model No. 0,338,006. This rod comprises a rod body 601 and an elastic connection portion 603 formed in the middle of the rod body 601. As shown in FIG. 3, the elastic rod is in various types, such as a semicircular ring, a coil spring, a bar type smaller then the diameter of the rod body, and the like.
These types provide the elastic connection portion for the rod in order that the rod body can be bent. This structure is capable of giving fluidity to the rod in a predetermined range between the pedicle segments. However, the rod structure results in a lose of the basic function of the spine correction since the elastic connection portion causes large movement. That is, the rod has to support and connect the spinal segments. In the above-mentioned structure, the pieces of the rod body are detachable so that they are freely movable in a given elastic range. Therefore, the secure connection between the spinal segments is not achieved.
Also, when the pedicle screws are out of the straight line, the rod structure has a problem in that it is difficult to connect the rod to the pedicle screw.
FIG. 4 shows a perspective view of the conventional transverse link 400 for preventing the pedicle from a minute movement.
The transverse link 400 comprises a fixed type housing 410 and a movable type housing 420 which are respectively hooked on both ends of the rod 250; a space bar 430 supported on the rod 250; and a set screw 440 connected with the fixed type housing 410 and the movable type housing 420 so that the space bar 430 is fixed to the rod 250.
The fixed type housing 410 and the movable type housing 420 respectively include half circle hooks 410a and 420a for connection on the rod 250; support holes 410b and 420b inserted into the both ends of the rod 250; screw holes 410c and 420c into which set screws 440 are inserted.
The hook 410a of the fixed type housing 410 is hooked on the rod 250 and then one end of the space bar 430 is inserted into the support hole 410b. The hook 420a of the movable type housing 420 is hooked on the rod 250 and then another end of the space bar 430 is inserted into the support hole 420b. The set screws 440 are joined to the screw holes 410c and 420c respectively, for securely tightening the rod 250 under the space bar 430.
In the structure of the transverse link 400, the inner diameter of the hook 410a is almost equal to that of the rod 250. Therefore, if one of the rods 250 is tilted or they are not in parallel, the transverse link 400 cannot comply with such a declination or unbalance of the rods 250. In case of the declination or unbalance of the rods 250, the support holes 410a and 420a of the fixed type housing 410 and movable type housing 420 are also unbalanced and thus the space bar 430 cannot be inserted into the support holes 410b and 420b. If strength is put on the space bar 430 for fixation on the support holes 410b and 420b, the position of the movable type housing 420 may be wrenched and distorted so that the rod 250 may be separated from the hook 420a of the movable type housing 420. In this case, even if the set screw 440 is joined to the screw hole 430, it is impossible to securely support the space bar 430 upon the rod 250. This problem in the transverse link 400 requires to take a long time for an operation on the surgery. In case where it is difficult to assemble the transverse link 400, it may be omitted; however, this will cause a defect of the surgery.